This invention relates to powder metallurgy composite materials of the type comprising grains of a relatively hard, abrasion resistant material, and a binder material for binding the grains together, and more particularly to a composite material having an improved binder exhibiting transformation-toughening properties. This invention also involves a method of heat-treating the composite material to improve its transformation-toughening properties, and a heat-treatable composite material having such improved transformation-toughening properties.
This invention involves an improvement over powder metallurgy composite materials of the type, such as that disclosed for example in U.S. Pat. No. 2,731,711, comprising grains of relatively hard, abrasion resistant material, such as tungsten carbide, and a binder material, such as a cobalt alloy, for binding the grains together. Such composite material, which is also referred to as a "cemented carbide" or more simply a "cermet", is widely used for cutting tools and as the cutting elements or so-called "inserts" for rolling cutter drill bits such as shown in U.S. Pat. No. 2,687,875, for use in drilling oil and gas wells.
While tungsten carbide with a cobalt alloy binder has been the standard composite material for use in inserts in the drill bit manufacturing industry for the past 30 years, this material (and more particularly the cobalt binder therefor) presents several significant disadvantages. Because cobalt is principally mined outside of the United States and has significant strategic importance militarily, the supply of cobalt to the United States drill bit manufacturing industry is vulnerable to disruptions and shortages. Moreover, because of the relatively small quantity of cobalt mined each year, the price of cobalt is subject to dramatic escalations during periods of high demand, such as occurred during the late 1970's. In addition, excessive wear and breakage of inserts, which reduce the useful life of drill bits, is attributable to the failure of the cobalt binder to hold the tungsten carbide grains together under the relatively high compressive loads (which may exceed 60,000 pounds) applied to the drill bit during drilling operations. More particularly, the limiting factor in the wear and life of drill bit inserts formed of conventional cermet material is the lack of adequate fracture toughness and resistance to fatigue crack growth, of the inserts made with a cobalt binder.